1. Field of the Invention
The present invention relates to a screw compressor provided with a capacity controlling slide valve.
2. Prior Art
A screw compressor provided with a capacity controlling slide valve 21 shown in FIGS. 7 and 8 has been heretofore well known (Japanese Patent Publication No. 7-62477). The slide valve 21 is coupled to a piston rod 23 of a hydraulic cylinder 22 arranged on the discharge side, and is provided for advance and (or) retreat along with a piston 25 through the piston rod 23 extending through a discharge port 24. As is known, the slide valve 21 comes in sliding contact with the internal surface of a casing opposite to, that is, on the back side of a pair of male and female screw rotors 26, 27, for advance and (or) retreat. The slide valve 21 operates to adjust the size of an opening in a diametral direction on the suction side of the screw rotors 26, 27. It is noted that only the opening 29 in the diametral direction of the one screw rotor 26 is shown in FIG. 8.
In the compressor, a side cover 31 on the discharge side formed with a discharge axial port 30 is provided with a projection 32 for controlling the diametral movement of the slide valve 21 (which is shown by the cross hatching in FIG. 7). The surface on the slide valve 21 side of the projection 32 comprises the guide surface which slidably contacts with the slide valve 21.
In the case of the aforementioned conventional compressor, compressed gas from the discharge port 24 moves into a fine clearance between the slide valve 21 and the internal surface of the casing 28 behind the slide valve 21. The pressure caused by the compressed gas presses on the back of the slide valve 21 as shown by the arrow A in FIG. 8. The diametral movement on the discharge side of the slide valve 21 can be controlled by the projection 32 with respect to the back pressure exerting on the slide valve 21. On the other side, however, the slide valve 21 cannot be prevented from being flexed towards the screw rotors 26, 27, on the suction side, as shown by the arrow B in FIG. 8. This poses problems in that the back pressure causes the contact between the slide valve 21 and the screw rotors 26, 27, the damage of the slide valve 21, and the screw rotors 26, 27 resulting therefrom, the abnormal noises, the lowering of performance, and the like.
The present invention has been accomplished in order to eliminate such problems as noted above with respect to prior art. It is an object of the present invention to provide a screw compressor which enables prevention of contact between a slide valve and screw rotors.